In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of a Device-to-Device (D2D) proximity service is discussed as a new function in Release 12 and later (see Non Patent Document 1).
The D2D proximity service (D2D ProSe) is a service enabling direct device-to-device communication within a synchronization cluster formed by a plurality of synchronized user terminals. The D2D proximity service includes a discovery procedure (Discovery) in which a proximal terminal is discovered and D2D communication (Communication) that is direct device-to-device communication.
In the D2D communication, the user terminal transmits (or receives) control information by using a control resource, and transmits (or receives) user data by using a data resource indicated by the control information. The control information is also referred to as SA (Scheduling Assignment), and the control resource is also referred to as SA resource.
Mode 1 (a first mode) and Mode 2 (a second mode) are proposed as the assignment mode of the control resource and the data resource. The Mode 1 is a base station-driven assignment mode in which the control resource and the data resource are designated from the base station. The Mode 2 is a user terminal-driven assignment mode in which the user terminal selects the control resource and the data resource from within a predetermined resource pool.